Manganese-Iridium Composite Oxide for Water-Splitting Catalyst, Manganese-Iridium Composite Oxide Electrode Material, and Methods for Producing the Same

Abstract

The present invention provides: a manganese-iridium complex oxide which is inexpensive, has high catalytic activity, and is used as a positive electrode catalyst for oxygen generation in water electrolysis; a manganese-iridium complex oxide electrode material; and production methods therefor.　The manganese-iridium complex oxide is characterized by having a metal-containing ratio of iridium (iridium / (manganese + iridium)) of 0.1-30 at%, and crystal plane spacing of at least 0.243±0.002 nm, 0.214±0.002 nm, 0.165±0.002 nm, and 0.140±0.002 nm. The manganese-iridium complex oxide electrode material is formed of an electrically conductive base material composed of fibers at least partially coated with the manganese-iridium complex oxide.

Description

Technical Field

[0001] The present invention relates to a manganese-iridium composite oxide for a water decomposition catalyst, a manganese-iridium composite oxide electrode material, methods for manufacturing them, and their uses. More specifically, it relates to a manganese-iridium composite oxide used as an oxygen evolution anode catalyst, a manganese-iridium composite oxide electrode material, and methods for manufacturing them in industrial water electrolysis carried out under alkaline conditions, neutral conditions, or acidic conditions, or in water electrolysis using a proton exchange membrane (PEM) electrolytic cell.

Background Art

[0002] Due to the depletion problem of fossil fuels and environmental pollution problems, attention has been focused on the utilization of hydrogen as a clean energy and its production methods. The water electrolysis method is one of the effective means for producing high-purity hydrogen gas from water by electrolysis. At this time, it is characteristic that oxygen evolution occurs simultaneously from the anode of the counter electrode. In order to efficiently progress the water decomposition reaction in the water electrolysis method, it is necessary to electrolyze while keeping the electrolysis voltage low by using an electrode catalyst with a low hydrogen overvoltage at the cathode and an electrode catalyst with a low oxygen overvoltage at the anode. Among these, as electrode catalyst materials excellent in low oxygen overvoltage at the anode, rare platinum group metals such as platinum (Pt), iridium (Ir), ruthenium (Ru), and compounds including oxides containing these elements have been proposed (Patent Documents 1, 2, Non-Patent Documents 1 to 3).

[0003] On the other hand, since the electrode catalyst composed of such platinum group metals is very expensive, the development of electrode catalysts using inexpensive transition metals has been promoted. For example, in recent years, transition metal materials composed of manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), etc. have been proposed (Patent Documents 3, 4, Non-Patent Documents 4 to 7).

[0004] However, the catalyst materials composed of transition metals proposed so far have a problem that they are significantly less active (higher oxygen overvoltage) compared to the platinum group metal-based electrode catalysts. That is, an oxygen evolution electrode catalyst material composed of an inexpensive transition metal and having a high catalytic activity comparable to that of platinum group metals such as Pt and Ir has not been realized. In response to such problems, manganese oxides having an oxygen evolution electrode catalytic activity equal to or higher than that of Pt have been found, but they do not reach the activity of Ir-based catalysts, which are considered to have the highest activity among platinum group metal elements, and further development has been awaited (Patent Document 5).

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Non-Patent Documents

[0006]

Non-Patent Document 1

[0007]

Non-Patent Document 2

[0008]

Non-Patent Document 3

[0009]

Non-Patent Document 4

[0010]

Non-Patent Document 5

[0011]

Non-Patent Document 6

[0012]

Non-Patent Document 7

Summary of the Invention

Problems to be Solved by the Invention

[0013] An object of the present invention is to provide a manganese-iridium composite oxide for a water decomposition catalyst, a manganese-iridium composite oxide electrode material, and methods for manufacturing them. More specifically, an object of the present invention is to provide a manganese-iridium composite oxide for a water decomposition catalyst, which is used as an oxygen generation anode catalyst material in industrial water electrolysis carried out under alkaline conditions, neutral conditions, or acidic conditions, or in water electrolysis using a polymer electrolyte membrane (PEM) type electrolytic cell, is inexpensive, and has high oxygen generation catalyst activity (hereinafter, may be referred to as the manganese-iridium composite oxide of the present invention), a manganese-iridium composite oxide electrode material for a water decomposition catalyst, and a method for producing them.

Means for Solving the Problems

[0014] As a result of intensive studies on a catalyst material used as an oxygen generation electrode catalyst for water electrolysis, the present inventors have found that a manganese-iridium composite oxide having a metal content ratio of iridium (iridium / (iridium + manganese)) of 0.1 atomic% or more and 30 atomic% or less and having crystal plane spacings of at least 0.243 ± 0.002 nm, 0.214 ± 0.002 nm, 0.165 ± 0.002 nm, and 0.140 ± 0.002 nm exhibits high oxygen generation electrode catalyst activity, and have completed the present invention. That is, the present invention is a manganese-iridium composite oxide for an oxygen generation electrode catalyst in water electrolysis, characterized in that the metal content ratio of iridium (iridium / (manganese + iridium)) is 0.1 atomic% or more and 30 atomic% or less and has crystal plane spacings of at least 0.243 ± 0.002 nm, 0.214 ± 0.002 nm, 0.165 ± 0.002 nm, and 0.140 ± 0.002 nm.

[0015] The present inventors have found that a manganese-iridium composite oxide electrode material in which the manganese-iridium composite oxide of the present invention coats at least a part of the fibers of a conductive base material exhibits particularly high oxygen generation electrode catalyst activity. That is, the present invention is a manganese-iridium composite oxide electrode material for an oxygen generation electrode comprising a conductive base material composed of fibers coated at least in part with the manganese-iridium composite oxide of the present invention.

Effects of the Invention

[0016] The manganese-iridium composite oxide of the present invention and the manganese-iridium composite oxide electrode material of the present invention exhibit high activity in industrial water electrolysis carried out under alkaline, neutral, or acidic conditions, or in water electrolysis using a PEM electrolytic cell, and act as an inexpensive and excellent anode catalyst for oxygen generation. Furthermore, the manganese-iridium composite oxide and the manganese-iridium composite oxide electrode material of the present invention exhibit extremely excellent durability as an oxygen generation electrode catalyst.

[0017] In addition, by adding carbon dioxide or the like to the electrolytic system using the manganese-iridium composite oxide electrode material of the present invention, the carbon dioxide or the like can be reduced at the cathode to produce hydrocarbon compounds (such as formic acid, formaldehyde, methanol, methane, ethane, propane, etc.).

Brief Description of Drawings

[0018]

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Mode for Carrying Out the Invention

[0019] Hereinafter, the present invention will be described in more detail. First, regarding the decomposition of water by electrolysis, a reaction in which the reaction field is in an acidic environment, such as PEM type water electrolysis, will be taken as an example for explanation. On the cathode catalyst, as shown in Equation 1, hydrogen is generated by the reaction of two protons and two electrons. 2H + + 2e - → H2… Equation 1

[0020] On the anode catalyst, as shown in Formula 2, oxygen is generated together with four electrons and four protons from two water molecules. 2H2O → O2+ 4H + + 4e - … Formula 2 And overall, as shown in Formula 3, it is a reaction in which two hydrogen molecules and one oxygen molecule are generated from two water molecules. 2H2O → 2H2+ O2… Formula 3

[0021] The oxygen generation reaction in the above Formula 2 is generally regarded as the rate-determining step of the overall reaction, and the development of a catalyst capable of promoting this reaction with a minimum amount of energy occupies an important position in this technical field. The present invention provides an oxygen generation electrode catalyst having a high water oxidation catalytic ability. In the manganese-iridium composite oxide of the present invention, the metal content ratio of iridium (iridium / (manganese + iridium)) is controlled to be 0.1 atomic% or more and 30 atomic% or less. When the metal content ratio of iridium is less than 0.1 atomic%, the characteristics are similar to those of a manganese oxide single catalyst. On the other hand, when the metal content ratio of iridium exceeds 30 atomic%, although the catalytic activity similar to that of iridium oxide is exhibited, since a large amount of iridium, which is a rare element, is used, it becomes extremely expensive and the cost performance is impaired. In order to exhibit excellent characteristics, the metal content ratio of iridium is preferably 0.2 atomic% or more and 27 atomic% or less, more preferably 0.2 atomic% or more and 20 atomic% or less, further preferably 0.3 atomic% or more and 10.5 atomic% or less, and particularly preferably 1 atomic% or more and 7 atomic% or less.

[0022] The XRD of the manganese-iridium composite oxide of the present invention has characteristic diffraction lines. In order from the diffraction lines on the low angle side, as the d value representing the crystal plane spacing, there are at least 0.243 ± 0.002 nm, 0.214 ± 0.002 nm, 0.165 ± 0.002 nm, and 0.140 ± 0.002 nm. There has been no reported example in the past of a substance having such an interplanar spacing with a manganese-iridium composite oxide, and it exhibits an X-ray diffraction pattern similar to that of epsilon-type manganese dioxide which contains a large amount of structural disorder due to microtwin formation.

[0023] By supporting the manganese-iridium composite oxide of the present invention on an electrode, the manganese-iridium composite oxide of the present invention can serve as an oxygen evolution electrode active material in water electrolysis, and can impart catalytic ability in the water decomposition reaction to the oxygen evolution electrode. By laminating an oxygen evolution electrode containing this oxygen evolution electrode active material, a polymer electrolyte membrane, and an electrode provided with a hydrogen evolution catalyst, a laminate is formed. Here, the polymer electrolyte membrane refers to, for example, a fluororesin-based cation exchange membrane, etc., and the hydrogen evolution catalyst refers to, for example, platinum fine particles, etc. In the present invention, by having this oxygen evolution electrode, a water electrolysis device is formed, and hydrogen can be produced by performing water electrolysis using this oxygen evolution electrode.

[0024] The manufacturing method of the manganese-iridium composite oxide of the present invention will be described below. The manganese-iridium composite oxide of the present invention can be manufactured, for example, by using a mixed solution of sulfuric acid - manganese sulfate - iridium salt as an electrolytic solution and electrolytically depositing it. Or, after first electrolytically depositing manganese oxide using a mixed solution of sulfuric acid - manganese sulfate, subsequently, iridium oxide can be electrolytically deposited on the manganese oxide by using a mixed solution of sulfuric acid - iridium salt.

[0025] Regarding the concentration of each component in these mixed solutions of sulfuric acid - manganese sulfate - iridium salt, mixed solution of sulfuric acid - manganese sulfate, and mixed solution of sulfuric acid - iridium salt, the sulfuric acid concentration is preferably controlled within a range exceeding 4 g / L and not exceeding 65 g / L, and more preferably 20 g / L or more and 50 g / L or less. The manganese concentration in the above mixed solution is not particularly limited as long as it is below the solubility, but is preferably 5 g / L or more and 50 g / L or less, and more preferably 10 g / L or more and 30 g / L or less. The iridium concentration in the mixed solution is also not particularly limited as long as it is below the solubility, but is preferably 0.1 g / L or more and 10 g / L or less, and more preferably 0.3 g / L or more and 5 g / L or less. Since the ratio of the manganese concentration to the iridium concentration in the mixed solution affects the metal composition of the manganese-iridium composite oxide deposited by electrolytic oxidation, the molar ratio of iridium / manganese in the mixed solution is preferably 0.002 or more and 0.05 or less, and more preferably 0.005 or more and 0.02 or less.

[0026] In order to maintain the component concentrations of the above mixed solution, it is effective to appropriately add manganese sulfate and iridium salts corresponding to the manganese and iridium consumed by electrolytic oxidation, or to continuously supply each solution of manganese sulfate and iridium salts, or the mixed solution. Note that the sulfuric acid concentration in the above mixed solution of sulfuric acid - manganese sulfate - iridium salt, mixed solution of sulfuric acid - manganese sulfate, and mixed solution of sulfuric acid - iridium salt is the value excluding the divalent anions (sulfate ions) of manganese sulfate.

[0027] In the electrolytic production method of the manganese-iridium composite oxide, the electrolytic current density is not particularly limited, but is preferably 0.2 A / dm 2 or more and 0.9 A / dm 2 or less. This makes it easier to electrolytically produce the manganese-iridium composite oxide of the present invention efficiently and stably. In order to more stably obtain the manganese-iridium composite oxide of the present invention, the electrolytic current density is preferably 0.3 A / dm 2 or more and 0.88 A / dm 2 or less, and more preferably 0.5 A / dm 2 or more and 0.8 A / dm 2 less is even more preferable. The electrolysis temperature can be exemplified as 93°C or more and 98°C or less. The higher the electrolysis temperature, the higher the electrolytic production efficiency of the deposited manganese-iridium composite oxide. Therefore, the electrolysis temperature is preferably at least more than 94°C.

[0028] The manganese-iridium composite oxide electrolytically produced on an electrode such as a pure titanium plate, after being peeled off from the electrode, undergoes coarse pulverization such as by a jaw crusher, and then is pulverized and adjusted to a predetermined average secondary particle size as a single manganese-iridium composite oxide by a roller mill, a vertical mill, a Ross mill, a jet mill, or the like. Next, the produced manganese oxide is dried using a flash dryer or the like after passing through a washing step and a neutralization step to remove residual electrolytic solution and the like. During this flash drying, fine powder of submicron manganese-iridium composite oxide by-produced by over-pulverization in the pulverization step can be collected and separated by a dust collector bag filter or the like. Further, a firing step at 200°C or higher and 500°C or lower may be performed to obtain the manganese-iridium composite oxide of the present invention.

[0029] Next, the manganese-iridium composite oxide electrode material of the present invention will be described. The manganese-iridium composite oxide electrode material of the present invention is composed of a conductive substrate composed of fibers at least partially coated with the manganese-iridium composite oxide of the present invention. In this case, the coating amount of the manganese-iridium composite oxide of the present invention is preferably 0.1 mg / cm 2 or more and 25 mg / cm 2 or less per geometric area of the conductive substrate. Here, the geometric area corresponds to the projected area of the conductive substrate, and the thickness of the substrate is not considered.

[0030] When the coating amount of the manganese-iridium composite oxide of the present invention is within the above range, although it also depends on the diameter and porosity of the fibers constituting the conductive substrate, it is coated in such a form that the manganese oxide is in an island shape or completely covers the outer surface of the fiber on the fiber, and the average coating thickness can be generally 25 μm or less. Note that since the manganese-iridium composite oxide coated on the fiber is composed of secondary particles, usually, the average coating thickness coincides with the average secondary particle size of the manganese-iridium composite oxide constituting it.

[0031] In the manganese-iridium composite oxide electrode material of the present invention, depending on the amount of the coated manganese-iridium composite oxide, the average thickness of the manganese-iridium composite oxide coating the fibers of the conductive substrate tends to increase. Among them, the coating amount of the manganese-iridium composite oxide is preferably 0.1 mg / cm 2 or more and 20 mg / cm 2 or less, more preferably 0.2 mg / cm 2 or more and 15 mg / cm 2 or less, and particularly preferably 0.5 mg / cm 2 or more and 12 mg / cm 2 or less. The thickness of the manganese-iridium composite oxide coating layer can also be obtained by subtracting the wire diameter thickness of the conductive fiber, which is a constituent unit of the conductive substrate, from the image of a scanning electron microscope (SEM), for example.

[0032] The manganese-iridium composite oxide electrode material of the present invention is obtained by electrochemically depositing the manganese-iridium composite oxide on a conductive substrate typified by carbon paper, a titanium mesh, or a platinum-coated titanium mesh, instead of the electrode substrate of the pure titanium plate, using the above-mentioned mixed solution of sulfuric acid-manganese sulfate-iridium salt. In this case, the manganese-iridium composite oxide is electrochemically deposited with a deposition amount of 0.1 mg / cm 2 or more and 25 mg / cm 2 or less per geometric area. If it is less than 0.1 mg / cm 2 per geometric area, the deposition amount of the manganese-iridium composite oxide is too small, and it may not adhere sufficiently to the substrate and may easily fall off, or the amount of catalyst may be too small and the performance may not be fully exhibited. If it exceeds 25 mg / cm 2 , the thickness of the manganese-iridium composite oxide coating layer is too thick, which may inhibit electron transfer to the conductive substrate, or the gaps between the conductive fibers of the conductive substrate may become narrow, restricting the movement of water and oxygen, which is a reaction product, and the catalytic performance may not be fully exhibited due to rate limitation. The conductive substrate is preferably, for example, a plate-shaped one obtained by molding or sintering conductive fibers such as carbon or titanium metal having a wire diameter thickness of 100 μm or less, with the thickness of the substrate being 1 mm or less. The porosity of the conductive substrate is preferably, for example, 40% or more, more preferably 50% - 90%. Here, the porosity is defined by the volume of the space portion without conductive fibers, etc. in the volume of the conductive substrate.

[0033] Before electrodepositing the manganese-iridium composite oxide on the conductive substrate, it is also effective to perform acid treatment with hydrochloric acid, sulfuric acid, nitric acid, oxalic acid, etc. to remove the passive film on the substrate surface and hydrophilize it. On the other hand, for the purpose of controlling the electrodeposition position of the manganese-iridium composite oxide in the conductive substrate or imparting important gas diffusion characteristics when actually used as an electrode for water electrolysis, it is also effective to immerse the conductive substrate in a dispersion liquid of a fluororesin, etc. to make it water-repellent. As the conditions for electrodepositing and coating the manganese-iridium composite oxide of the present invention on the conductive substrate, for example, select the ranges of the sulfuric acid concentration, manganese concentration, iridium concentration, electrolysis current density, electrolysis temperature, etc. of the mixed solution of sulfuric acid - manganese sulfate - iridium salt, the mixed solution of sulfuric acid - manganese sulfate, and the mixed solution of sulfuric acid - iridium salt as described above, perform the electrolysis for a range of 5 minutes to 120 minutes, and after the electrolysis is completed, wash with water and dry to obtain the manganese-iridium composite oxide electrode material of the present invention. When one side of the conductive substrate is shielded with a resinous film, etc. during the electrodeposition of this manganese-iridium composite oxide, while preferentially electrodepositing the electrodeposition film of the manganese-iridium composite oxide only on one side, it is also possible to intentionally segregate the manganese-iridium composite oxide on the other side with almost no electrodeposition of the manganese-iridium composite oxide.

[0034] In addition, as a post-treatment for the manganese-iridium composite oxide electrode material of the present invention, it is effective to perform either acid immersion or heating, or both acid immersion and heating. The post-treatment by acid immersion is obtained, for example, by immersing the manganese-iridium composite oxide electrode material in sulfuric acid of 0.5 mol / L to 5 mol / L for about 30 minutes to 2 hours, followed by washing with water and drying. The post-treatment by heating is obtained, for example, by heating the manganese-iridium composite oxide electrode material at 180°C to 500°C for 30 minutes to 8 hours in an air or nitrogen atmosphere. That is, it can also be obtained by heating at 180°C to 300°C for 30 minutes to 2 hours in an air or nitrogen atmosphere, but it is more preferable to heat at a temperature exceeding 300°C and not exceeding 500°C for more than 2 hours and not exceeding 8 hours in an air or nitrogen atmosphere. It is presumed that these post-treatments increase the adhesion between the manganese-iridium composite oxide and the conductive fiber, or increase the crystallinity of the manganese-iridium composite oxide.

[0035] By laminating the manganese-iridium composite oxide electrode material of the present invention, a polymer electrolyte membrane, and an electrode provided with a hydrogen generation catalyst, a laminate is formed. In the present invention, by having the manganese-iridium composite oxide electrode material of the present invention, a water electrolysis device is formed, and hydrogen can be produced by performing water electrolysis using this manganese-iridium composite oxide electrode material.

Examples

[0036] Hereinafter, the present invention will be described in detail with reference to Examples and Comparative Examples, but the present invention is not limited to these Examples. <Analysis of Metal Concentrations in Mixed Solutions of Sulfuric Acid-Manganese Sulfate-Iridium Salt, Sulfuric Acid-Manganese Sulfate, and Sulfuric Acid-Iridium Salt> The mixed solutions of sulfuric acid-manganese sulfate-iridium salt, sulfuric acid-manganese sulfate, and sulfuric acid-iridium salt were diluted, and the concentrations of manganese and iridium elements were quantitatively measured using ICP-AES (Optima 8300 manufactured by PerkinElmer). <SEM Surface Observation and Composition Analysis of Manganese-Iridium Composite Oxide and Manganese-Iridium Composite Oxide Electrode Material> Using a general SEM-EPMA apparatus (Hitachi S-4500 and Horiba EX-250 (EDS)), surface state observation and elemental analysis (detectable elements: B to U) were performed. The acceleration voltage was set to 15 kV.

[0037] <Calculation of Interplanar Spacing (d Value) of Manganese-Iridium Composite Oxide and Manganese-Iridium Composite Oxide Electrode Material by XRD Measurement> Using a general X-ray diffractometer (Rigaku Ultima IV), diffraction lines of manganese-iridium composite oxide and manganese-iridium composite oxide electrode material were measured. CuKα radiation (λ = 1.5405 Å) was used as the radiation source. The measurement mode was step scan, the scan speed was 4.0000° per minute, the step width was 0.02°, and the measurement range was from 5° to 80° in terms of 2θ. The diffraction lines of the obtained XRD pattern were Gaussian-processed to obtain the 2θ of the peak top, and the d value was calculated from Bragg's equation (nλ = 2dsinθ, n = 1) to obtain the interplanar spacing. <Measurement of Electrodeposition Amount of Manganese-Iridium Composite Oxide> The electrodeposition amount of manganese-iridium composite oxide was measured according to the following method. Before electrodepositing the manganese-iridium composite oxide, the weight (1) of the base material (such as an electrode base material or a conductive base material like titanium) was measured in advance with a balance. After electrodeposition, the weight (2) of the base material on which the manganese-iridium composite oxide was electrodeposited was measured with a balance. The electrodeposition amount of the manganese-iridium composite oxide was determined from the difference between weight (2) and weight (1) (weight (2) - weight (1)).

[0038] <Construction of a PEM-Type Electrolyzer for Evaluating Oxygen Generation Electrode Catalyst Characteristics> The construction of a PEM electrolyzer using an electrode material of a conductive substrate deposited with a manganese-iridium composite oxide catalyst was carried out as follows. An electrode material (flat mesh shape: 3 cm × 3 cm) was used as the working electrode, and as the catalyst for the counter electrode, a 20 wt% platinum-supported carbon catalyst (20% Platinum on Vulcan XC-72, Item# PTC20-1, Fuel Cell Earth) was used. A conductive catalyst ink was prepared, applied to carbon paper, and air-dried to fabricate the counter electrode. As the electrolyte membrane, a Nafion membrane (Nafion 117, manufactured by Sigma-Aldrich) was used. The electrolyte membrane was washed and protonated (pretreatment) by boiling in 3% hydrogen peroxide solution, pure water, 1 M sulfuric acid aqueous solution, and then pure water for 1 hour each. Next, the electrolyte membrane was sandwiched between the catalyst-coated surfaces of the working electrode and the counter electrode, and a membrane / electrolyte assembly (MEA) was fabricated by hot pressing at 135 °C and a molding force of 600 kg for 10 minutes using a hot press machine (A-010D, manufactured by FC-R&D). This MEA was attached to the casing of a PEM electrolyzer 1 (3036, manufactured by FC-R&D) or a PEM electrolyzer 2 (WE-4S-RICW, manufactured by FC Development Co., Ltd.) through two stainless steel meshes (#100) to improve the adhesion even during electrolysis operation.

[0039] <Electrochemical Measurement 1 Measurement of Current-Voltage Curve (Using PEM Electrolyzer 1)> To evaluate the water oxidation catalytic ability in an actual device, the current-voltage curve was measured at an operating temperature of 25 °C using PEM electrolyzer 1 constructed using an electrode material of a conductive substrate deposited with a manganese-iridium composite oxide. In this measurement, a two-electrode system with only the working electrode and the counter electrode was used, and the current-voltage curve was measured by gradually increasing the applied voltage. Pure water was supplied to the PEM electrolyzer. The rate of voltage increase was set to 5 mV / s while paying attention to make it easy to distinguish the voltage at which the current rises. In the measurement results of the current-voltage curve, the higher the current at the same voltage, the higher the oxygen electrode catalytic activity.

[0040] <Electrochemical Measurement 2 Measurement of Current-Voltage Curve (Using PEM Electrolyzer 2)> To evaluate the water oxidation catalytic ability in the actual device, measurements of the current-voltage curve were performed at an operating temperature of 25 °C or 80 °C using a PEM electrolyzer 2 constructed with an electrode material of a conductive substrate on which a manganese-iridium composite oxide was deposited. In this measurement, a two-electrode system with only the working electrode and the counter electrode was used, and the current-voltage curve was measured by gradually increasing the applied voltage. Pure water was supplied to the PEM electrolyzer. The rate of increase in voltage was set to 5 mV / s while paying attention to making it easy to distinguish the voltage at which the current rises. <Measurement of Electrochemical Measurement 3 Electrolysis Voltage Stability (Using PEM Electrolyzer 2)> To evaluate the stability of the water oxidation catalytic ability in the actual device, measurements of the electrolysis voltage were performed at an operating temperature of 80 °C using a PEM electrolyzer 2 constructed with an electrode material of a conductive substrate on which a manganese-iridium composite oxide was deposited. In this measurement, a two-electrode system with only the working electrode and the counter electrode was used, and while maintaining the current density applied between the two electrodes at 0.5 A / cm 2 the time change of the electrolysis voltage was measured. Pure water was supplied to the PEM electrolyzer. The smaller the time change of the electrolysis voltage, the higher the performance of stably maintaining the oxygen electrode catalytic activity, that is, the so-called durability.

[0041] Example 1 Electrolysis was carried out in an electrolyzer containing a mixed solution of 35 g / L sulfuric acid, 51.3 g / L manganese sulfate, and 2.0 g / L potassium hexachloroiridate (K2IrCl6) to electrodeposit a manganese-iridium composite oxide on carbon paper (TGP-H-060, Toray). The electrolysis was carried out at an electrolysis current density of 0.7 A / dm 2 and an electrolysis temperature of 95 °C for 60 minutes. After the electrolysis, it was washed with water and air-dried, and cut into a size of 3 cm × 3 cm to prepare an electrode material. Composition analysis and XRD measurement of this manganese-iridium composite oxide electrode material were performed, and the results are shown in Table 1.

[0042] Examples 2 to 12 A mixed solution was prepared and electrolyzed in the same manner as in Example 1, except that the manganese sulfate concentration, the type of iridium salt, the substrate type, and the electrolysis temperature were those shown in Table 1. The manganese-iridium composite oxide was electrodeposited on carbon paper (TGP-H-060, manufactured by Toray Industries, Inc.) or a platinum-coated Ti mesh (ADL-414302-5056, manufactured by Fushi Kaihatsu Co., Ltd.). The electrolysis was carried out at an electrolysis current density of 0.7 A / dm 2 for 60 minutes. After the electrolysis was completed, the electrode was washed with water, air-dried, and cut into a size of 3 cm × 3 cm to prepare an electrode material. Composition analysis and XRD measurement of these manganese-iridium composite oxide electrode materials were performed, and the results are shown in Table 1 below, as well as in FIGS. 1 and 2. In addition, as shown in the SEM photograph of the appearance of the electrode material in Example 12 (FIG. 4), a manganese-iridium composite oxide coating the carbon fibers (FIG. 3) constituting the carbon paper was confirmed.

[0043] Comparative Example 1 Electrolysis was carried out in an electrolytic cell containing a mixed solution of 32 g / L sulfuric acid and 75 g / L manganese sulfate, and manganese oxide was electrodeposited on carbon paper (TGP-H-060, manufactured by Toray Industries, Inc.). The electrolysis was carried out at an electrolysis current density of 0.68 A / dm 2 at an electrolysis temperature of 94°C for 16 minutes. After the electrolysis was completed, the electrode was washed with water, air-dried, and cut into a size of 3 cm × 3 cm to prepare an electrode material. XRD measurement of this manganese oxide electrode material was performed, and the results are shown in Table 1 below, as well as in FIGS. 1 and 2.

[0044] As shown in FIGS. 1 and 2, the XRD pattern of the example, excluding the diffraction lines shown by the carbon paper substrate, has almost no diffraction line near 22 degrees and the other four main diffraction lines are shifted to the low-angle side compared to Comparative Example 1. In addition, for the manganese oxide electrode material of Comparative Example 1, a PEM electrolytic cell was constructed according to the method of <Construction of PEM Electrolytic Cell for Evaluation of Oxygen Evolution Electrode Catalyst Characteristics>, and the oxygen evolution electrode catalyst characteristics were evaluated according to <Electrochemical Measurement 1 Measurement of Current-Voltage Curve (PEM Electrolytic Cell)> and <Electrochemical Measurement 2 Measurement of Current-Voltage Curve (PEM Electrolytic Cell 2)>. The results are shown in Table 2 and FIGS. 5 to 9 and FIGS. 12 to 13 below.

[0045] Comparative Example 2 A PEM electrolytic cell was constructed using a conductive catalyst ink prepared by mixing 20 wt% platinum-supported carbon catalyst (20% Platinum on Vulcan XC-72, Item# PTC20-1, Fuel Cell Earth) and a diluted Nafion dispersion (10 wt.% aqueous solution, 527106-25ML, manufactured by Sigma-Aldrich), and the oxygen evolution electrode catalyst characteristics were evaluated according to <Electrochemical Measurement 1 Measurement of Current-Voltage Curve (PEM Electrolytic Cell)>. The results are shown in Table 2 and FIG. 5 below.

[0046] Comparative Example 3 A PEM electrolytic cell was constructed using a commercially available iridium oxide catalyst (manufactured by Elyst), and the oxygen evolution electrode catalyst was evaluated according to <Electrochemical Measurement 1 Measurement of Current-Voltage Curve (PEM Electrolytic Cell)>. The characteristic evaluation results are shown in Table 2 and FIG. 5 below.

[0047] Examples 13 to 20 Electrolysis was carried out in an electrolytic cell containing a mixed solution of 2.0 g / L of potassium hexachloroiridate (K2IrCl6) under the same conditions as in Example 1 except that the manganese sulfate concentration, the type of iridium salt, the substrate type, and the electrolysis temperature were the same as those in Table 2, and the manganese-iridium composite oxide was electrodeposited on carbon paper (TGP-H-060, Toray) or a Ti mesh coated with platinum (ADL-414302-5056, manufactured by FCC Development). The electrolysis was carried out at an electrolysis current density of 0.7 A / dm 2, and it was carried out for 60 minutes. After the electrolysis was completed, it was washed with water and air-dried, and then cut into a size of 3 cm × 3 cm to prepare an electrode material. Using this electrode material, a PEM electrolyzer was constructed according to the method of <Construction of a PEM Electrolyzer for Evaluation of Oxygen Evolution Electrode Catalyst Characteristics>, and the oxygen evolution electrode catalyst characteristics were evaluated according to <Electrochemical Measurement 1 Measurement of Current-Voltage Curve (PEM Electrolyzer 1)> or <Electrochemical Measurement 2 Measurement of Current-Voltage Curve (PEM Electrolyzer 2)>. The results are shown in Table 2 and Figure 6 below.

[0048] Examples 21 to 22 Electrolysis was carried out in an electrolytic cell containing a mixed solution of 2.0 g / L of potassium hexachloroiridate (K2IrCl6) under the same conditions as in Example 1 except that the manganese sulfate concentration, the type of iridium salt, the substrate type, and the electrolysis temperature were those shown in Table 2, and the manganese-iridium composite oxide was electrodeposited on carbon paper (TGP-H-060, manufactured by Toray Industries, Inc.) or a Ti mesh coated with platinum (ADL-414302-5056, manufactured by Fushi Kaihatsu Co., Ltd.). The electrolysis was carried out at an electrolysis current density of 0.7 A / dm 2 , and it was carried out for 60 minutes. After the electrolysis was completed, it was washed with water and air-dried, annealed at 230 °C for 2 hours, and then cut into a size of 3 cm × 3 cm to prepare an electrode material. Using this electrode material, a PEM electrolyzer was constructed according to the method of <Construction of a PEM Electrolyzer for Evaluation of Oxygen Evolution Electrode Catalyst Characteristics>, and the oxygen evolution electrode catalyst characteristics were evaluated according to <Electrochemical Measurement 2 Measurement of Current-Voltage Curve (PEM Electrolyzer 2)>. The results are shown in Table 2, Figure 7, and Figure 9 below. Furthermore, in the electrode material of Example 21, using the PEM electrolyzer constructed according to the method of <Construction of a PEM Electrolyzer for Evaluation of Oxygen Evolution Electrode Catalyst Characteristics>, the time change of the electrolysis voltage was measured according to <Electrochemical Measurement 3 Measurement of Electrolysis Voltage Stability (PEM Electrolyzer 2)>. The results are shown in Figure 10.

[0049] As shown in FIGS. 5 to 9, the manganese-iridium composite oxide and the manganese-iridium composite oxide electrode material of the present invention have a structure that is theoretically capable of achieving high energy conversion efficiency, and it has been revealed that they exhibit high oxygen evolution electrode catalyst activity comparable to that of commercially available iridium oxide catalysts even in a PEM type electrolytic cell where non-noble metalization of the catalyst is desired. Also, as shown in FIG. 10, it has been revealed that the manganese-iridium composite oxide and the manganese-iridium composite oxide electrode material of the present invention exhibit extremely excellent durability as an oxygen evolution electrode catalyst.

[0050] [Table 1]

[0051] [Table 2]

[0052] Example 23 Electrolysis was carried out in an electrolytic cell containing a mixed solution of 35 g / L sulfuric acid, 2.71 g / L manganese sulfate, and 0.48 g / L potassium hexachloroiridate (K2IrCl6), and the manganese-iridium composite oxide was electrodeposited on a Ti mesh (ADL-414302-5056, manufactured by FCC Development Co., Ltd.) coated with platinum. The electrolysis was carried out at 94°C with an electrolysis current density of 0.7 A / dm 2 , for 60 minutes. After the electrolysis was completed, it was washed with water and air-dried, heat-treated at 350°C for 5 hours, and then cut into a size of 3 cm × 3 cm to prepare an electrode material. Using this electrode material, a PEM type electrolytic cell was constructed according to the method of <Construction of a PEM type electrolytic cell for evaluation of oxygen evolution electrode catalyst characteristics>, and the oxygen evolution electrode catalyst characteristics were evaluated according to <Electrochemical measurement 2 Measurement of current-voltage curve (PEM type electrolytic cell 2)>. The results are shown in Tables 3, 4, and 5 below, and FIG. 12.

[0053] Example 24 In an electrolytic cell containing a mixed solution of 35 g / L sulfuric acid and 54.4 g / L manganese sulfate, at 94°C, the electrolysis current density was 0.7 A / dm2 Electrolysis was carried out for 10 minutes, and manganese oxide was electrodeposited on a Ti mesh (ADL-414302-5056, manufactured by C&D Development Co., Ltd.) coated with platinum. Subsequently, in an electrolytic cell containing a mixed solution of 35 g / L sulfuric acid and 0.16 g / L potassium hexachloroiridate (K2IrCl6), at 94 °C, the electrolysis current density was 0.7 A / dm 2 Electrolysis was carried out for 60 minutes to electrodeposit iridium oxide on the manganese oxide. After the electrolysis was completed, it was washed with water and air-dried, heat-treated at 400 °C for 5 hours, and then cut into a size of 3 cm × 3 cm to prepare an electrode material. The SEM photograph of this electrode material is shown in Fig. 11. Using this electrode material, a PEM electrolytic cell was constructed according to the method of <Construction of a PEM electrolytic cell for evaluation of oxygen evolution electrode catalyst characteristics>, and the oxygen evolution electrode catalyst characteristics were evaluated according to <Electrochemical measurement 2 Measurement of current-voltage curve (PEM electrolytic cell 2)>. The results are shown in Table 3, Table 4, and Table 5 below and Fig. 13. As shown in Fig. 12 and Fig. 13, the manganese-iridium composite oxide and the manganese-iridium composite oxide electrode material of the present invention have a structure in which the energy conversion efficiency is inherently high. By performing appropriate heat treatment as a post-treatment, they show particularly good oxygen evolution electrode catalytic activity, and even in a PEM electrolytic cell where non-noble metalization of the catalyst is desired, it has been clarified that they show a high oxygen evolution electrode catalytic activity comparable to that of a commercially available iridium oxide catalyst.

[0054] [Table 3]

[0055] [Table 4]

[0056] [Table 5]

Industrial Applicability

[0057] The manganese-iridium composite oxide of the present invention and the manganese-iridium composite oxide electrode material have a high oxygen evolution electrode catalytic activity comparable to that of conventional noble metal catalysts. Therefore, by using them as an oxygen evolution anode catalyst in industrial water electrolysis carried out under alkaline or neutral conditions or in water electrolysis using a PEM electrolytic cell, it is possible to obtain hydrogen and oxygen at extremely low production costs.

[0058] Furthermore, by introducing carbon dioxide into the reaction system such as the above-mentioned water electrolysis, the carbon dioxide etc. can be reduced at the cathode to produce hydrocarbon compounds (such as formic acid, formaldehyde, methanol, methane, ethane, propane, etc.).

[0059] The entire contents of the specification, claims, drawings, and abstract of Japanese Patent Application No. 2020-54589 filed on March 25, 2020 are hereby incorporated by reference and made a part of the disclosure of the specification of the present invention.

Claims

1. A manganese-iridium composite oxide, wherein the metal content ratio of iridium (iridium / (manganese + iridium)) is 0.1 atomic% or more and 30 atomic% or less, and having crystal plane spacings of at least 0.243 ± 0.002 nm, 0.214 ± 0.002 nm, 0.165 ± 0.002 nm, and 0.140 ± 0.002 nm. A manganese-iridium composite oxide characterized by the above.

2. A manganese-iridium composite oxide electrode material, characterized by comprising a conductive substrate composed of fibers at least partially coated with the manganese-iridium composite oxide according to Claim 1.

3. The manganese-iridium composite oxide is coated at 0.1 mg / cm or more and 25 mg / cm or less per geometric area of the conductive substrate. ２ or more ２ The manganese-iridium composite oxide electrode material according to claim 2, which is coated as described above.

4. The manganese-iridium composite oxide electrode material according to Claim 2 or 3, wherein the conductive substrate is composed of carbon, titanium, or titanium coated with platinum.

5. A laminate having the manganese-iridium composite oxide electrode material according to any one of Claims 2 to 4 and a polymer electrolyte membrane.

6. A method for producing a manganese-iridium composite oxide electrode material according to any one of Claims 2 to 4, wherein the manganese-iridium composite oxide according to Claim 1 or 2 is electrodeposited by electrolysis of a mixed solution containing sulfuric acid - manganese sulfate - iridium salt on the fibers constituting the conductive substrate. A manufacturing method characterized by the above.

7. A method for producing a manganese-iridium composite oxide electrode material according to any one of Claims 2 to 4, wherein after electrolysis with a sulfuric acid - manganese sulfate mixed solution on the fibers constituting the conductive substrate, electrolysis is performed with a mixed solution containing sulfuric acid - iridium salt to coat the manganese-iridium composite oxide according to Claim 1. A manufacturing method characterized by the above.

8. A method for producing a manganese-iridium composite oxide electrode material according to Claim 6 or 7, characterized by performing heat treatment.

9. The method for producing a manganese-iridium composite oxide electrode material according to Claim 8, wherein the heat treatment is performed at 180°C to 500°C for 30 minutes to 8 hours.

10. The manganese-iridium composite oxide according to claim 1 is electrodeposited with a deposition amount of 0.1 mg / cm or more and 25 mg / cm or less per geometric area. The manufacturing method according to any one of claims 6 to 9. ２ or more and ２ less.

11. An oxygen evolution electrode active material in water electrolysis containing the manganese-iridium composite oxide according to Claim 1.

12. An oxygen evolution electrode containing the oxygen evolution electrode active material according to Claim 11.

13. A laminate having the oxygen evolution electrode according to Claim 12 and a polymer electrolyte membrane.

14. A water electrolysis device having the manganese-iridium composite oxide electrode material according to any one of claims 2 to 4 or the oxygen generation electrode according to claim 12.

15. A method for producing hydrogen by water electrolysis using the manganese-iridium composite oxide electrode material according to any one of claims 2 to 4 or the oxygen generation electrode according to claim 12.

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